Sudoku_Solver.java

class Solution {
    public boolean isSafe(char[][] board,int row,int col,int num)
    {
         //row and col
        for(int i=0;i<board.length;i++)
        {
            if(board[i][col]== (char)(num + '0')){
                return false;
            }
            if(board[row][i] == (char)(num + '0')){
                return false;
            }
        }
        
        //grid
        int sr = (row/3) * 3;
        int sc = (col/3) * 3;
        for(int i=sr;i<sr+3;i++){
            for(int j=sc;j<sc+3;j++){
                if(board[i][j]==(char)(num + '0')){
                    return false;
                }
            }
        }
        return true;
    }
    public boolean helper(char[][] board,int row,int col)
    {
        if(row == board.length){
            return true;
        }
        
        int nrow = 0;
        int ncol = 0;
        if(col!=board.length-1){
            nrow = row;
            ncol = col + 1;
        }
        else{
            nrow = row + 1;
            ncol = 0;
        }
        if(board[row][col] != '.'){
            if(helper(board,nrow,ncol)){
                return true;
            }
        }
        else{
            for(int i=1;i<=9;i++){
                if(isSafe(board,row,col,i)){
                    board[row][col] = (char)(i +'0');
                    if(helper(board,nrow,ncol)){
                        return true;
                    }else{
                        board[row][col] = '.';
                    }
                }
            }
        }
        return false;
    }
    public void solveSudoku(char[][] board) {
        helper(board,0,0);
    }
}